Transposons have been shown to play major roles in spontaneous mutation in Drosophila and other eukaryotes. In chromosomes, the insertion or excision of these mobile elements disrupts gene function at the site of insertion and may create deletions if excision is not precise. There are a variety of ways that transposons interfere with gene function at and around the site of their insertion. We are investigating how transposons may act as enhancers or repressors of neighboring genes through position effect over distances of several kilobases. Additionally, recombination between two copies of a transposon situated at different sites in a chromosome or in homologs produces a variety of chromosomal rearrangements, depending on the orientation of the paired mobile elements. We are studying rearrangements produced by these types of ectopic recombination to determine the molecular structures at their breakpoint junctions and to learn more about how transposons mediate these types of asymmetrical exchanges. We are also studying the conditions that promote insertion/excision of the retrotransposon gypsy.